RFR: 8087557: [Win] [Accessibility, Dialogs] Alert Dialog content is not fully read by Screen Reader

[Ambarish Rapte]

Accessibility is not implemented for JavaFX dialogs. This change is to implement the accessibility on windows platform.
Without this fix : On Windows platform, content of Dialog are not read by Windows Narrator or JAWS.
With this fix:
1. Windows narrator reads the dialog correctly
2. JAWS fails to read the header text. : This will be handled separately 
3. Behavior of VoiceOver on MacOS remains unchanged : This will be handled separately.

Verification:
1. Run HelloAlert toy app, with fix.
2. Start Windows Narrator
3. Click on the different buttons to show different Dialogs
=> Observe that Narrator reads all the content correctly.

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Commit messages:
 - unit test
 - Add DIALOG role

Changes: https://git.openjdk.org/jfx/pull/873/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=873&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8087557
  Stats: 36 lines in 5 files changed: 34 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/873.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/873/head:pull/873

PR: https://git.openjdk.org/jfx/pull/873